A challenge of survivable networking with optically transparent switching is that failure of a fiber span is not easily and quickly localized to the span at fault. In an “opaque” network, every optical line signal is electronically processed for switching and failures are inherently identified within their span of origin. But in an optically transparent network, loss-of-light will propagate along the length of the affected path. To achieve low cost, optically transparent switching nodes typically do not have fast-acting abilities to sense and isolate such failures. Methods are available following the failure to sectionalize the fault, but they are generally slower than required for activation of a protection switching response. However, a promising recent idea for fault localization takes a distributed approach in the work of Zeng, Huang and Vukovich [1] [2] with the elegant idea of an m-cycle cover of the graph.
The idea behind m-cycles is that if a network graph has a cycle cover in which the set of cycles covering each span differs by at least one cycle, then when a span fails, the span can be uniquely identified by the combination of covering cycles which display an alarm state. This assumes one signal monitor per cycle, which can be placed at any node on the cycle. This central idea was studied in the Ph.D. thesis by Zeng [2]. As so far considered, the idea is to use the fault localization as input to a separate protection or restoration scheme. The m-cycle scheme itself is agnostic about the survivability mechanism employed and could be used as the activating input for any span-protection scheme. In [2] the main focus is to find a wavelength-based cycle cover that maximizes the degree of fault localization using the minimum total length of cycles and/or number of cycle monitors. A branch and bound algorithm was developed to produce near-optimal solutions for the min-cost m-cycle cover problem.
p-Cycles predate m-cycles but at least structurally can be observed to be also cycles of pre-connected spare channels. Although from that point on m-cycles and p-cycles differ completely in their purpose, operation, and design as so far considered in the literature. p-Cycles operate for traffic protection, assuming the failed span is known. When one of the spans on a p-cycle fails, the cycle loops back around the body of the cycle similar to a BLSR ring. For any such “on cycle” failure there is one protection path available per unit of spare capacity in the p-cycle. But when a failure span “straddles” the cycle, two protection paths are available. p-Cycles are now a well established approach to fast and efficient survivable networking. See references such as [3] and [4]. In this document we determine that p-cycles may also serve as m-cycles, thus a group of span-protecting p-cycles as a set may also serve as an m-cycle cover of the graph for fault localization.